This invention relates to electric contact pieces in general and more particularly to a method for generating a graphite free, solderable surface on a shaped part for electric contacts, which part consists of silver and graphite particles up to 5% by weight finely and uniformly distributed therein.
Shaped parts of electric contact pieces are preferably made of silver, since this metal has a high electric and thermal conductivity. In contacts which are closed accompanied by chattering and arc formation, however, the contact pieces tend to get welded together, so that these contact pieces can then be separated from each other only by the exertion of force (the so-called welding force). One therefore often prefers shaped parts in which graphite particles are finely and uniformly distributed. Such shaped parts have the additional advantage that only a small contact resistance occurs at the contact surface of the shaped parts. Methods for producing such shaped parts of a composite, silver and graphite containing material are known in powder metallurgy.
However, it is a consequence of this reduced tendency to weld that shaped parts of this composite material cannot be joined to support metals, for instance, current leads, without difficulty. Thus, melted solder does not wet the graphite containing silver surface, so that silver soldering is not possible even if the usual fluxes are employed.
So as to make a reliable joint with the support metal possible in spite of this, the shaped parts are made as two layer pieces with a solderable layer. The second layer may, for instance, be pure silver or a silver alloy. In the case of very thin shaped parts, however, the overall thickness of the part is increased thereby. Thus, for a shaped part, for instance, the thickness of which is between 0.5 and 1 mm, the solderable layer must be between 0.2 and 0.3 mm for tolerance reasons and therefore already occupies a considerable part of the volume of the entire shaped part.
In another known method, the graphite particles are burned out by a heat treatment in an oxidizing atmosphere in that area of the finished shaped part which is later to serve as the soldering surface. Thereby, a graphite free layer is now produced in the originally single layer shaped part, the thickness of which is subject to large fluctuations when manufactured in the usual production furnaces. Thus only a tolerance of .+-.0.04 mm was obtained under conditions which were set for a burned out layer of 0.05 mm. This layer thickness must be chosen at least so thick, however, so that the melted solder penetrates sufficiently into the soldering surface during the soldering and a durable joint is obtained. This method can therefore likewise be used only for thicker shaped parts.